The invention relates to means and methods that enable the safe and reliable reduction of nitrogen oxides (NOx) emissions while permitting a diesel or other lean-burn engine to operate efficiently.
Diesel and lean-burn gasoline engines provide advantages in fuel economy, but produce both NOx and particulates during normal operation. When primary measures (actions that affect the combustion process itself, e.g., exhaust gas recirculation and engine timing adjustments) are taken to reduce one, the other is usually increased. Thus, combustion conditions selected to reduce pollution from particulates and obtain good fuel economy tend to increase NOx.
Current and proposed regulations challenge manufacturers to achieve good fuel economy and reduce particulates and NOx. Lean-burn engines will be necessary to achieve the fuel economy objective, but the high concentrations of oxygen in the exhaust renders typical exhaust gas catalyst systems ineffective for reducing NOx.
SCR (selective catalytic reduction) has been available for years in some contexts for reducing NOx. To date, however, SCR has depended on the use of ammonia, which has safety problems associated with its storage and transport. Urea is safer, but has not been practical for many SCR applicationsxe2x80x94particularly mobile NOx sourcesxe2x80x94due to the difficulty in converting it from a solid or an aqueous form to its active gaseous species, typically NHi and HNCO radicals.
There is a current need for a safe, economical and effective answer to the problems associated with urea SCR, particularly for mobile diesel and other lean-burn engines.
Where SCR catalysts are employed to limit NOx emissions from diesel engines, one has to deal with either the dangers of ammonia or a risk of fouling the catalysts under most conditions. In this regard, see R. J. Hulterman; A Selective Catalytic Reduction Of NOx from Diesel Engines Using Injection Of Urea, Ph.D. thesis, September 1995. Hulterman describes a number of technical challenges including clogging of atomizers, decomposition problems and system dynamics. Similarly, in SAE Paper No. 970185, entitled xe2x80x9cTransient Performance of a Urea DeNOx Catalyst for Low Emissions Heavy-Duty Diesel Enginesxe2x80x9d, it is indicated that the injection nozzles must protected from undue heat.
The limited attempts to use urea SCR for diesel engines have required the use of large pyrolization chambers or other devices following the point of urea introduction into the exhaust, as disclosed in U.S. Pat. No. 5,431,893, to Hug, et al. Equipment of this type highlights the known problems with urea. Once introduced into diesel exhaust, urea takes time to break down and may cause nozzle plugging as conventionally introduced and still as proposed in that disclosure. To protect the catalyst from fouling, Hug, et al., propose bulky equipment. In addition, this disclosure highlights the necessity of maintaining the urea solution at a temperature below 100xc2x0 C. to prevent hydrolysis prior to passage through the nozzle. They propose the use of moderate urea pressures when feeding the urea and find it necessary to have alternative means to introduce high-pressure air into the feed line when it becomes plugged, The nozzles employed by Hug, et al., are apparently capable of producing moderately-fine sprays, the dispersion of which is aided by auxiliary air, but the droplets are still large enough to require a large pyrolization channel.
Each of the points of caution by these references about the difficulties of using urea with SCR systems, especially for mobile sources, points to the trouble the art has had and continues to have.
The art is awaiting the development of a process and apparatus that would permit the use of urea in an SCR process simply, reliably, economically and safely.
It is an object of the invention to provide a safe, reliable SCR system for reducing NOx emissions from an internal combustion engine.
It is another object of the invention to eliminate the safety problems associated with the storage and handling of ammonia for mobile uses.
It is another object of the invention to provide a simple, robust, SCR system capable of rapid response time in order to meet transient conditions prevailing in diesel engines.
It is another object of the invention to enable a use of urea for SCR that avoids wetting of or solids deposition on the catalyst.
It is a yet further and more specific object of the invention to enable the use of urea in an SCR system that enables modulation for exhaust gas temperatures between 180 and 600xc2x0 C.
It is a yet further and more specific object of the invention to enable the use of urea in an SCR system which enables integration with EMS.
It is yet another specific object of the invention to provide a simple mechanical device for accomplishing the above objects and preferably to enable close coupling of the reagent injection means and the SCR catalyst.
It is another specific object of the invention to decompose urea for use in an SCR system, without the production of HNCO or CO in significant quantities.
These and other objects are achieved by the present invention, which provides an improved process and apparatus for NOx reduction. The process, in one of its aspects, comprises: supplying an aqueous urea solution at a preset, elevated pressure, e.g., of from about 50 to 600 psi, into a vessel having an inlet and an outlet; heating the urea solution in the vessel at a temperature above the hydrolysis temperature for the urea solution to produce ammonia; releasing ammonia from the vessel; introducing the ammonia into the exhaust gas at an exhaust gas temperature effective for SCR, preferably from about 190 to about 650xc2x0 C.; and passing the exhaust gas containing the ammonia through an SCR reactor.